An amplifying circuit for amplifying high frequency signals is used in radio communications equipment. Furthermore, a FET (field effect transistor) can be used for the amplifying element. FIG. 2 shows a conventional FET amplifying circuit 20 as an example. A GaAs FET is for an amplifying element FET a shown in FIG. 2. The amplifying element FET a shown in FIG. 2 has a gate connected to voltage dividing resistor Rg3 via a negative supply (−Vg), a drain connected to an inductor L2 via a positive supply (+Vd), and a grounded source. A bias voltage Vgs1 is applied across the gate and source so that a drain current Ids1 flows to place the amplifying element FET a into a predetermined operating class (for example, class AB). The amplifying element FET a, which has been placed in a predetermined operating class by being applied with the bias voltage Vgs1 in this manner, amplifies a high frequency signal that has been input by an input terminal IN via a capacitor C1 and outputs an amplified high frequency signal from an output terminal OUT via a capacitor C2.
Variations in the characteristics of semiconductor devices, such as the amplifying element FET a, can be found within individual devices, as well as among multiple devices. In an example of the former, the amplifying element FET a exhibits variations in the characteristic of the drain current Ids1 with respect to the bias voltage Vgs1 and in the change in the drain current Ids1 with respect to temperature change, while, in an example of the latter, similar variations in characteristic often occur among FETs from different production lots.
To account for production variations, the voltage divider resistor Rg3 (bias circuit) in the bias circuit of a conventional FET amplifying circuit 20, to which is applied the bias voltage Vgs1, is normally adjusted prior to shipment to conform to a predetermined operating class (for example, class AB). However, to compensate for changes in the drain current Ids1 due to temperature changes, it is still necessary to provide a temperature sensitive element, such as a thermistor, at a suitable location on the voltage divider resistor Rg3 and to adjust the temperature sensitive element in accordance with the characteristic of the individual amplifying element FET a, so that the bias voltage Vgs1 will adjust to the changes in the drain current Ids1.
With respect to this issue, it is desirable to detect the drain current Ids1 of the amplifying element FET a and control the bias voltage Vgs1 to place the circuit in a predetermined operating class. However, since the amount of the drain current Ids1 varies in accordance with the input of the high frequency signal in an operating class, such as class AB, detecting the drain current Ids1 of the amplifying element FET a to maintain the bias voltage Vgs1 at a predetermined operating class is difficult.
It is an object of the present invention to realize a FET bias circuit for applying the bias voltage for a predetermined operating class to the amplifying element FET a of the FET amplifying circuit.